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    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
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    <script>
        //只定义，没赋值
        console.log(a)//undefined
        console.log(typeof a)//undefined
        a=10//后赋值 变量提升 只提升定义 不提升赋值  所以a是undefined 不是10
    </script>
</body>
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